1. Field of the Invention
This invention relates to improved compositions and processing methods for piezoelectric ceramic/polymer composites, and related devices utilizing such composites.
2. Description of the Prior Art
Piezoelectric ceramics and piezoelectric polymers both provide useful sensor materials. Unfortunately, each offers limited advantages. The large longitudinal (d.sub.33) piezoelectric charge coefficient (typically about 200 pC/N) and high dielectric constant (typically about 1,000) of piezoelectric ceramics make them nearly ideal for applications where the frequency is well over 500 kHz. They also provide a large piezoelectric response and good electrical impedance match with the interface electronics. Hence, piezoelectric ceramics are widely used in medical ultrasonic image applications where the operational frequency is typically between 1 MHz and 50 MHz. In contrast with the situation at the above high frequencies, the sound pressure exerted on a piezoelectric sensor is effectively hydrostatic at frequencies below 100 kHz, because the wavelength of sound in media such as air and water is much larger than the dimension of the sensor. In such case the piezoelectric ceramics have a problem. This problem arises since the hydrostatic charge coefficient (d.sub.h) equals d.sub.33 +2d.sub.31 when d.sub.31 =d.sub.32, and d.sub.33 is typically about equal to -2d.sub.31 and d.sub.h =d.sub.33 +2d.sub.31. Hence, the hydrostatic charge coefficient is typically much smaller than d.sub.33. Another problem with piezoelectric ceramics is that they have a very high acoustic impedance, because of a high density and high elastic modulus. This leads to a mismatch in acoustic impedance between sensors and media such as air or water. In addition, the high mechanical quality of the piezoelectric ceramic causes ringing within sensors, and hence a damping layer is sometimes necessary.
On the other hand, piezoelectric polymers are flexible and have an acoustic impedance that is closer to water and human tissue than piezoelectric ceramics. However, they have a low piezoelectric charge coefficient (d.sub.33 typically about 25 pC/N) and a low dielectric constant (typically about 10). This results in a low piezoelectric response and low capacitance, which causes difficulties for matching the sensor electrical impedance with the interface electronics. Also, the current piezoelectric polymers cannot be operated at very high temperatures (typically above 70.degree. C.), since they depole.
The above analysis indicates that piezoelectric ceramics and piezoelectric polymers cannot meet various conflicting requirements for the broadest range of transducer applications. For this reason, composite materials consisting of piezoelectric ceramics and polymers have been investigated, which potentially combine the advantages of the piezoelectric ceramics and piezoelectric polymers. Here, it important to differentiate between fundamentally different types of composites that have been described by Newnham, Skinner, and Cross (Mat. Res. Bull. 13, 1978, pp. 525-536). These composites are indicated according to the connectivity of the ceramic and polymer components. Zero-, one-, two-, and three-dimensional connectivities are denoted by the indices 0, 1, 2, and 3. The index of the ceramic component is listed first, and this index is followed by that of second component. Hence, a ceramic-polymer composite in which both phases have three-dimensional connectivity is called a 3-3 composite. Also, a ceramic-polymer composite containing a continuous polymer matrix and parallel ceramic rods that transverse the composite is called a 1-3 composite. Likewise, a ceramic-polymer composite in which isolated ceramic particles are embedded in a three-dimensionally continuous polymer phase is called a 0-3 composite. Because of uncertainties that typically exist regarding the degree of association between ceramic powder particles, an index of zero is used herein to refer to this component, independent of such association. While such definition differs from one entirely based on the connectivity of possibly aggregated powder particles, it permits convenient description of results from the prior art--where the connectivity of the powder particles is often unestablished.
A problem with many of the prior art sensor technologies of composites relates to the achievable piezoelectric coefficients and the values of electrical loss, which are best considered by describing figures of merit. One figure of merit for piezoelectric composites for long range and low frequency applications is how much electrical energy can be generated (per unit volume of piezoelectric material) in response to one unit change in pressure. This figure of merit is called the hydrophone power figure of merit. It can be expressed in terms of the hydrostatic piezoelectric voltage and charge piezoelectric constants (g.sub.h and d.sub.h, respectively) as g.sub.h d.sub.h. When this figure of merit is exclusively used, all piezoelectric materials are assumed to have the same dielectric loss, which is an invalid assumption unless they have such a small dielectric loss that the contribution of dielectric loss to noise can be ignored. Typically, the dielectric loss of piezoelectric materials can vary from 0.001 to above 0.1. Most composite materials of the prior art have significant dielectric loss that generates large sensor-self noise--Johnson noise that is directly proportional to dielectric loss. Hence, a particularly useful sensor figure of merit includes dielectric loss. After taking into account the dielectric loss (tan.delta.), the figure of merit can be rewritten as g.sub.h d.sub.h /tan.delta.. This figure of merit determines the sensor signal-to-noise ratio, which is the ultimate measurement of sensor materials performance.
The 1-3 piezoelectric composites have been extensively investigated. For example, such 1-3 composites are fabricated using PZT rods that are extruded, sintered, and hot-pressed. These rods are aligned in parallel within a mold. After backfilling polymer into the mold, the composite fabrication is completed cutting plates perpendicular to the rod direction. Although the properties of such composites are attractive and reproducible, this fabrication process is time consuming and costly. Such 1-3 composites can offer a figure of merit as high as 20.times.10.sup.-13 m.sup.2 /N without the use of any mechanical transformer (Pohanka, et al., in "Electronic Ceramics", Levinson, L. M. Ed. Marcel Dekker Inc., New York, 1987, pp.45-147). Also, 1-3 composites made from polyurethane foam can offer a g.sub.h d.sub.h value of 200.times.10.sup.-13 m.sup.2 /N. However, the porosity of such polyurethane foams causes problems in practical applications.
The 1-3 composites do not provide a cost effective alternative to 0-3 composites. Also, the 0-3 piezoelectric ceramic particle/polymer composites of the prior art typically have serious problems because of high dielectric loss, moderate figures of merit, and a sensitivity dependence on hydrostatic pressure loading. U.S. Pat. No. 4,624,796 to Newnham et al. (and Proceedings of 1986 IEEE International Symposium on Applied Ferroelectrics, 1986, pp. 285-289) claimed a 0-3 composite with a reasonably high g.sub.h d.sub.h value of 38.times.10.sup.-13 m.sup.2 /N, but the reported dielectric loss of the composite was as high as 0.1. Banno et al. (Japanese J. Appl. Phys. 26, 1987, pp. 153-155) demonstrated that the 0-3 piezoelectric ceramic particle/polymer composite could achieve a g.sub.h d.sub.h value as high as about 50.times.10.sup.-13 m.sup.2 /N, but the dielectric loss was also quite high (0.03-0.05). Han et al. (J. Am. Ceram. Soc. 74, 1991, pp. 1699-1701) reported similar composites with a comparable g.sub.h d.sub.h value and dielectric loss of 0.04 to 0.08. Waller et al. (J. Amer. Ceram. Soc.72, 1989, pp. 322-324) and Tandon et al. (Journal of Materials Science Letters 12, 1993, pp. 1182-1184) have described the fabrication and evaluation of 0-3 composites of piezoelectric ceramics in a host polymer that is non-piezoelectric. Also, Giniewicz et al. have described (U.S. Pat. No. 4,624,796) fabrication of 0-3 ceramic-polymer composites of a ceramic comprising a PbTiO.sub.3 -BiFeO.sub.3 solid solution (with partial substitution of iron by manganese) in an insulating polymer. Ceramic powder loadings claimed for obtaining the highest sensitivity sensors were in the range of 50-75 volume percent, and preferably 60-75 volume percent. Despite such high volume percentages of ceramic (60%), the maximum observed d.sub.h and d.sub.h g.sub.h did not exceed 35 pC/N and 35.times.10.sup.-13 m.sup.2 /N, respectively. In related work, Sa-Gong et al. have reported (U.S. Pat. Nos. 4,944,891 and 5,043,622) fabrication of 0-3 composites of an insulating matrix polymer, a piezoelectric ceramic, and a conducting additive (such as carbon, silicon, or germanium) to enhance the pole-ability of the ceramic. U.S. Pat. No. 5,043,622 shows that a PZT particle/epoxy composite has a low g.sub.h d.sub.h value of 4.times.10.sup.-13 m.sup.2 /N and a dielectric loss as high as 0.08.
The problem of high loss (i.e., high tan.delta.) piezoelectrics of the prior art results in a high sensor Johnson noise, which is proportional to (tan.delta.)/.omega.C (where C is the capacitance of the sensor and co is the angular frequency). The Johnson noise can be decreased by either reducing the dielectric loss of the material or increasing the sensor capacitance, while keeping the sensor operational frequency range constant. Increasing the sensor capacitance by increasing electrode area (keeping thickness constant) is equivalent to increasing the volume of the sensor materials used. The use of a larger volume of sensor materials is often undesirable because the sensor can become too bulky, heavy, and costly. Thus, the preferred way to make better piezoelectric sensors is to use sensor materials having a high d.sub.h g.sub.h value and a low dielectric loss. The present invention describes how to make 0-3 piezoelectric composite materials having both very high d.sub.h g.sub.h and g.sub.h d.sub.h /tan.delta. figures of merit, This means that the piezoelectric composites of the present invention provide much higher volumetric capacities for the generation electrical energy from applied stresses, as well as dramatically improved signal-to-noise capabilities for sensors, compared with 0-3 composites of the prior art.